1. Field of the Invention
The invention relates to a process for preparing a solution of cellulose in water-containing tertiary amine N-oxides via preparation of a mixture containing cellulose, tertiary amine N-oxides and water and subsequent reduction of the water content of this mixture.
2. Description of Related Art
A known technique of preparing cellulose solutions, as described in, e.g., U.S. Pat. No. 4,246,221 or in WO 83/04415, provides that the cellulose is first suspended in a mixture of tertiary amine N-oxide (solvent) and water (non-solvent). In this case water is present in excess, i.e., in a higher percentage than the cellulose solution should exhibit thereafter. The swelling of the cellulose is improved by adding water. This facilitates the adsorption of tertiary amine N-oxide on the cellulose and promotes therefore the subsequent dissolving process of the cellulose.
The dissolving process is initiated through the removal of excess quantities of the non-solvent water from the suspension by heating and application of vacuum. In this case it is important to keep the thermal load on the cellulose as small as possible, that is, to keep the retention time of the cellulose suspension or cellulose solution under high temperatures as short as possible in order to avoid at least to a large degree the undesired degradation of the cellulose and the tertiary amine Noxide.
This technique allows the preparation of relatively highly concentrated cellulose solutions, which can be formed particularly well into foils, membranes, filaments, hollow filaments, some with semi-permeable properties, or in general into cellulose-based formed objects.
From WO 83/04415, among other things, the discontinuous production of cellulose solutions according to the above described technique is known. According to this document the cellulose, tertiary amine N-oxide, water and, if necessary, stabilizers are mixed to a suspension in a mixer, which is operated batch-by-batch, whereby the water content of the suspension amounts to 33.3%. The suspension, while being subjected to the effect of the mixing aggregate in the mixer and to a vacuum between 90 mbar and 180 mbar, is heated to temperatures between 80.degree. C. and 100.degree. C., and by this process excess water is removed. Hereby, the cellulose is dissolved; the water content of the finished cellulose solution amounts to 9%. The retention time of the cellulose suspension or cellulose solution in the mixer at the stated temperatures amounts to between 2 and 4 hours.
From WO 83/04415, among other things, the discontinuous production of cellulose solutions according to the above described technique is known. According to this document the cellulose, tertiary amine N-oxide, water and, if necessary, stabilizers are mixed to a suspension in a mixer, which is operated batch-by-batch, whereby the water content of the suspension amounts to 33.3%. The suspension, while being subjected to the effect of the mixing aggregate in the mixer and to a vacuum between 90 mbar and 180 mbar, is heated to temperatures between 80.degree. C. and 100.degree. C., and by this process excess water is removed. Hereby, the cellulose is dissolved; the water content of the finished cellulose solution amounts to 9%. The retention time of the cellulose suspension or cellulose solution in the mixer at the stated temperatures amounts to between 2 and 4 hours.
Additionally, the increased volume of such mixers requires an increased gradient between the temperature of the mixture contained in the mixer and the heating medium streaming through the heating jacket of the mixer in order to guarantee the heat input necessary for the evaporation of the excess water. As a consequence the portion of cellulose suspension or cellulose solution situated at the heated outer wall of the mixer undergoes an increased temperature load, which in turn causes increased damage to the cellulose.
EP-A-356 419 describes a process for the continuous preparation of cellulose solutions in water-containing tertiary amine N-oxide solutions. According to this document a suspension of cellulose, tertiary amine N-oxide and water is continuously spread and transported as a layer or film over a heated surface, whereby excess water is evaporated from the suspension through heat application and reduced pressure and the suspension is transformed into a solution.
The described production of the cellulose solutions is carried out in costly apparatus composed of cylindrical mixing vessels which can be evacuated and heated having a centrally arranged rotor which is equipped with adjustable stirring blades. The complexity of such apparatus, whose construction corresponds to a thin-film evaporator, has high production costs and additionally is highly susceptible to faults. A flexible adaptation of the parameters of the apparatus is only possible to a restricted degree. Additionally, an increased energy expenditure is necessary for the operation, mainly for the relatively fast running rotors in order to obtain the necessary shearing of the thinnest practicable layers or films.